1. Field of the Invention
This invention relates to an optical system having a stable resonator and more particularly to an optical system having a resonator which is used as an optical system for irradiating or generating ultraviolet rays (for example, ultraviolet light having a wavelength of 400 nm or shorter).
2. Description of Related Art
Heretofore, optical systems having a stable resonator have been involved in the problem of high order transverse mode generation. In detail, the existence of, for example, a scattering source in a resonator results in generation of a high order transverse mode in the resonator due to slight scattering from the scattering source. Because such high order transverse mode competes with the longitudinal mode, such generation of a high order transverse mode results in reduced longitudinal mode output in the resonator. Irregular generation of a high order transverse mode results in unstable longitudinal mode output in the resonator. Therefore, such generation of a high order transverse mode has been a problem in obtaining a stable longitudinal mode, particularly a constant stable longitudinal mode with time. Any optical system having at least one optical part other than mirrors in a stable resonator has been involved in such problem.
This problem is particularly more serious for wavelength conversion optical systems having an external resonator. In wavelength conversion using an external resonator (refer to M. Oka and S. Kubota, Jpn. J. Appl. Phys. Vol. 31 (1992) pp. 513 and M. Oka et. al., in Digest of Conference on Laser and Electron-Optics (OSA. Washington D.C. 1992), paper CWQ7), an external resonator is provided with an optical crystal for functioning as a wavelength conversion element, slight scattering due to impurities contained in the optical crystal and surface causes generation of a high order transverse mode in the resonator. The high order transverse mode results in reduced or unstable fundamental wave output in the resonator, and results in a reduced or unstable wavelength-converted harmonic. As described herein above, the above-mentioned generation of a transverse mode has been a problem in obtaining stable harmonic output in wavelength conversion optical systems having an external resonator.
The above-mentioned problem of the prior art will be described hereinunder with reference to drawings. For example, when a fundamental wave having a wavelength of 532 nm is converted to ultraviolet light having a wavelength of 266 nm using an external resonator, an external resonator having the structure shown in FIG. 7 has been used.
In FIG. 7, characters 1 to 3 represent high reflectance mirrors having ultra high reflectance, for example, as high as 99.95% or higher at the wavelength of 532 nm, character 4 represents an input (incident) mirror having a reflectance of, for example, 99% at the wavelength of 532 nm, and character 5 represents a non-linear optical crystal BBO, namely a wavelength conversion element provided with a low reflectance film of, for example, as low as 0.1% or lower at the wavelength of 532 nm. The high reflectance mirror 3 is placed on a VCM (refer to in the above-mentioned J. J. A. P) namely a positioning device though not shown in the drawing, and controlled by, for example, a servo driving system. As described herein above, the external resonator is composed of components shown with the characters 1 to 5.
A fundamental wave (wavelength of 532 nm) is irradiated onto the external resonator, amplified between the mirrors, and the amplified fundamental wave is converted to a second harmonic (wavelength of 266 nm in this case) by the non-linear optical crystal 5 (BBO). The second harmonic is shown with the arrow 7 in FIG. 7.
When the wavelength is converted as described herein above, a transverse mode due to scattering can be generated, particularly in the case of an external resonator as shown in FIG. 7, slight scattering due to photorefractive characteristics of the non-linear optical crystal 5 (BBO) increases to result in generation of the high transverse mode. As a result, the output of the fundamental wave having a wavelength of 532 which contributes to wavelength conversion is reduced.
The output of the second harmonic generated by wavelength conversion is formulated by the following equation: EQU P.sub.2.omega.=.gamma..sub.SH (P.omega.).sup.2
Wherein, P.omega. represents the output of the fundamental wave irradiated onto the non-linear optical crystal 5 (BBO), P.sub.2.omega. represents the output of the second harmonic generated by wavelength conversion using the non-linear optical crystal 5, and .gamma..sub.SH represents a constant called a non-linear conversion factor determined from the crystal length of the non-linear optical crystal 5, the wavelength of the fundamental wave, the spot size, and a focusing parameter.
From this equation it is obvious that the output of the second harmonic P.sub.2.omega. decreases with decreasing output of the fundamental wave P.omega., and the output of the second harmonic becomes unstable with irregular generation of a high order transverse mode.
Actually in wavelength conversion using the external resonator shown in FIG. 8, the second harmonic output (axis of ordinate, mW) exhibits unstable behavior with time (abscissa).